New research undermines 'RNA world' of early evolution

Did RNA evolve DNA, or did the two emerge simultaneously? Until recently, most scientists suggested the former, but new research undermines the theory, inspiring a team of scientists to suggest a second, alternative RNA-DNA origin story.

The idea that primordial Earth hosted an RNA-only chapter of evolution prior to the emergence of DNA is known as the "RNA world" hypothesis. A series of chemical reactions begat RNA, which eventually synthesized proteins and enzymes, which in turn combined with RNA to produce DNA.

The theory holds water for a variety of reasons, but not least of all because DNA looks like a completed version of RNA. DNA's structure resembles a ladder, with nucleobase pairs as the rungs and sugar molecules forming the two sides. RNA looks like just one side of a ladder.

The RNA world theory implies an intermediary stage, with RNA nucleotides as the rungs and DNA sugar molecules as the sides -- so called "heterogeneous" strands.

But when scientists at the Scripps Research Institute built DNA-RNA chimeras, the result was completely unstable.

"We were surprised to see a very deep drop in what we would call the 'thermal stability,'" Ramanarayanan Krishnamurthy, associate professor of chemistry at TSRI, said in a news release.

In the human body, when RNA accidentally finds its way onto a DNA ladder, enzymes are quickly called to the scene to rectify the mistake. It's clear evolution favors homogeneous DNA and RNA molecules.

It's unlikely repair enzymes were around when the first life forms first emerged, which means there must have been some biochemical mechanism keeping DNA and RNA apart.

Researchers believe their latest research, published this week in the journal Angewandte Chemie International Edition, supports the alternative theory that RNA and DNA evolved simultaneously.

"Even if you believe in a RNA-only world, you have to believe in something that existed with RNA to help it move forward," said Krishnamurthy. "Why not think of RNA and DNA rising together, rather than trying to convert RNA to DNA by means of some fantastic chemistry at a prebiotic stage?"